Follow signs for Wasdale Head from the A595. At the side of Wast Water there are one or two reasonably good sized gravel lay-by's almost right on the waters edge, and no signs saying 'no overnight stays'.
The scenery is absolutely breathtaking, and it's in the middle of nowhere so little chance of being bothered in the middle of the night.
Also there is a large triangular grassy parking area at wasdale head I have stayed there a few times with know problems (as long as you quiet and clean there never is!)